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Old 5th March 2005, 06:24 PM   #1
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Default another way to turn a pentode or tetrode into a triode

I've been meaning to get a web page with this information up for some time (the JoeList post was made in 2000 ). I'll be very interested in people's take on the technique.

http://www.t-linespeakers.org/tubes/triode-trick.html

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Old 5th March 2005, 11:23 PM   #2
SY is offline SY  United States
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What's the purported advantage of dropping the screen DC voltage? That's really the same question I have about the 43% DC stuff, too. Sure you can do that, but why? Why not BOOST the screen voltage relative to the plate, while you're at it?
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Old 6th March 2005, 01:51 AM   #3
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Yeah, I can get the same effect by turning down B+ about 20%; screen voltage controls maximum plate current and thus power output. Bad enough you have to drag the poor thing down in parallel with the plate, let alone deprive it of more voltage!

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Old 6th March 2005, 05:33 AM   #4
rdf is offline rdf  Canada
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How about this? In an output stage the maximum rated screen voltage sets the anode voltage limit of a triode-connected pentode. Dropping it a constant voltage permits a higher B+ and higher maximum output power while (possibly?) retaining triode-like AC operation. What I don't understand is the reasoning behind only dropping it ~30 volts. Of course, this completely ignores the question of the effect on the plate curves.
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Old 6th March 2005, 05:43 AM   #5
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The maximum screen voltage is routinely exceeded in triode operation of pentodes. Fritz Langford-Smith (of RDH fame) did it when adapting the Williamson design for 807s and found no appreciable decrease in valve life. Telefunken specifies 250V max for the LS50, but then allows 800V under triode operation (or so I think - the datasheet was in German). It doesn't seem internal arcing to or from g2 is particularly likely - especially considering how the grids and their voltage levels are arranged. I suppose internal arcing around the base could be possible, though.

I think the greater risk is exceeding the maximum power dissipation of the screen - so the screen current must be monitored (this is easily done by measuring the voltage across the screen stopper resistor).

Nonetheless, exceeding maximum ratings still makes me nervous...
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Old 6th March 2005, 08:43 AM   #6
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Quote:
Originally posted by rdf
How about this? In an output stage the maximum rated screen voltage sets the anode voltage limit of a triode-connected pentode. Dropping it a constant voltage permits a higher B+ and higher maximum output power while (possibly?) retaining triode-like AC operation. What I don't understand is the reasoning behind only dropping it ~30 volts. Of course, this completely ignores the question of the effect on the plate curves.
This is possible, for instance 6W6, 807 and more significantly 6146 have drastically different screen and plate ratings (or heck, any sweep tube), but there is a flaw: if screen follows plate in parallel at AC, take for example Vp = 600V, Vs = 300V: if plate drops only 300V, screen is already at zero and the tube is past cutoff in this assumption. This is greater than 300V saturation voltage! I figure, if reduced DC screen voltage is the goal, the AC signal must be reduced in respective ratio. (This could be done with a voltage divider to ground feeding a cathode follower supplied by the B+ rail.

But again, in the examples I cited, all have triode voltage limits far above the screen voltage alone; 6W6 is in fact rated for vertical output which requires withstanding a peak above 1kV, cutoff. (6V6 too, although it has no problem with screen voltage.)

The reality is that, at any given point in triode mode, the actual pentode-style operating point is above the high-Is saturation knee. Because screen falls with plate voltage, it is never greater; on the other hand, screen voltage is high, but the plate is high as well.

But anyway, I'm trailing on with long winded, tired replies....

Tim
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Old 6th March 2005, 07:29 PM   #7
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Bill Pearl explains the rational, courtesy of Ken Gilbert:



Quote:
From http://ken-gilbert.com/techstuff/tri...ntodesbts.html

-----------------------------------------------------
Date: Wed, 10 May 2000 13:51:27 -0600
Subject: [JN] Pet/Tets in"Triode" - was Quiet???
From: "PEARL Cust Serv"

Hi:
> OK Hugh.
>
> Will someone PLEASE explain what the zener thing for strapping pentodes as
> triodes is all about?
>
> I asked once and so far, everyone's ignored me.
>
> Regards,
> Paul

Hi Paul, All:

Basically, this is just a method of "level shifting" the screen to a lower DC potential than the plate and doing so thru a network with *very* low Z at audio frequencies.

Why is this a good thing?

1 - Well if you look at the plate curves for pentode/beam tetrode operation you see that plate current is pretty much a function of screen voltage and that's why most amplification stages that employ these devices also regulate the screen supply. Otherwise plate current can be affected by the changes in screen voltage that will inevitably occur with the changes in screen current that accompany signal. The object being of course to provide a low-Z or "stiff" screen supply.

2 - While it's the desired case for the turns of the screen grid lie within the "electron shadow" created by the negative charge on the control grid, this seldom turns out to be achieved in practice. Metal ceramic transmitting tetrodes being a notable and *very* interesting exception ie. 4CX-125, -250, -350 and their ilk.

When the screen is less than perfectly shadowed, its ability to attract electrons passing thru its accelerating, positive field on their way to the plate increases dramatically. Current at voltage equals power and consequently the screen must be able to effectively dissipate this, otherwise it will melt down eventually.

3 - The more nearly equal the screen and plate voltages, the greater the screen current, and the greater the changes in its value with signal.

4 - In "triode connected" pet/tets, this issue is typically "addressed" by the insertion of 100 to 1K ohms in series with the screen. While this *will* reduce the screen's voltage wrt to the plate and with that, the screen's current and consequent requirement for dissipation, it is far from an ideal fix.

5 - Referring back to (1), it will be seen that plate current is a function of screen voltage. If the screen is intercepting electronsdestined for the plate and if, triode-like, its current flow varies with it voltage it can be seen that that the potential on the screen will vary with the voltage drop across the resistor connecting it to the plate.

6 - By this action, the effective impedance of the entire B+ supply is driven up by the insertion of a screen resistor. This action is the cause of the "softish," "mellow" sound universally reported when pet/tet/UL amps are "triode-connected"

7 - The solution for this is to "level-shift" the screen to a lower DC level - thus limiting its dissipation - and to bypass the level-shifting circuitry with a *low* impedance ie. a *very* good, high value, low voltage cap of the BlackGate, Cerafine, OsCon variety.

8 - 1N821s work well for this as they are sonically reasonably benign and completely bypassed down to very low audio frequencies.

So, there you go Paul. I hope this is satisfactory . . . .

That will be $US250.00 . . . . Cash, VISA or MC :>)

Best regards,
Bill - PEARL, Inc.
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Old 6th March 2005, 07:37 PM   #8
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I don't believe that #3 is correct. And if you're losing 30V out of, say, 400V, that's not changing the screen dissipation dramatically. I'd still like to see some backup measurements showing the advantage of this approach- Bill is waving his hands nicely, but it's still a hand-wave.
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Old 6th March 2005, 07:58 PM   #9
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Most of the amplifiers use 100ohms for the screen of 6550. I tried to go down. I found that 22ohms is enough for 6550s, and KT88 I tried. Reducing the series resistor, reduce the effect mentioned by Bill.

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Old 6th March 2005, 08:37 PM   #10
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rdf -- thanx for pulling that post out... i've updated my page.

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